Pin Sphinx to 8.x to fix documentation build.

Our custom Sphinx extension `myst_docstring`, which lets us write
doc-strings in Markdown instead of reStructuredText, is not compatible
with Sphinx 9, for which the Autodoc extension "has been substantially
rewritten, and there may be some incompatible changes in edge cases,
especially when extensions interact with autodoc internals."

See the release notes:
https://www.sphinx-doc.org/en/master/changes/9.0.html#release-9-0-0-released-nov-30-2025

That's exactly what our extension does though. The mitigation
recommended in the release notes, to set
`autodoc_use_legacy_class_based = True` in `conf.py` does not seem to
help.

Author: john-hen

pyproject.toml

@@ -36,7 +36,7 @@ test = [
     'codecov      >= 2',
 ]
 docs = [
-    'Sphinx      >= 8',
+    'Sphinx      >= 8, < 9',
     'MyST-parser >= 1',
     'Furo        >= 2024',
 ]